CORE Loss TESTS 27 



field at a little above normal speed, and then suddenly 

 cut off the driving power and observe the deceleration; 

 then do the same thing with full field on the machine. 

 In the first case the deceleration is due to the retarding 

 force to friction and windage and in the second 

 case to these factors plus core loss. Readings of the 

 speed of the rotating parts should be taken at sufficiently 

 frequent intervals to obtain a uniform and reliable 

 curve. A set of these curves is shown in Figs. 8 and 9. 

 With the aid of these curves, together with a "running 

 light" test or a calculation of the kinetic energy of the 

 rotating parts, a determination of the value of the core 

 loss, and also of the friction and windage, is readily 

 made. The following is a brief derivation of the formulae 

 used in calculating such results by either method. 

 If M =mass. 



W = weight. 

 v = linear velocity at radius of gyration. 



co = angular velocity. 



5 = speed r.p.m. corresponding to angular velocity. 



g =32.2 ft. per sec. per sec. 

 r = radius of gyration. 



Wr 2 = flywheel effect. 



E = power at speed s and time T 



EI = power at speed Si and time T\ 



E 2 = power at speed 5 2 and time T 2 



Then -^ 2 = average power from T\ to J\ 



The kinetic energy of a rotating body at any instant 

 is equal to 



W . W , , 2ws 



i Mv 2 = v 2 = (rco) 2 where co = 



The energy consumed in decelerating from 



Wr 2 

 co! to co 2 = ^(co^-coy =0.00017 Wr 2 (Si 2 -S 2 2 ) 



~^6 



foot pounds. 



vSince the average power during the interval multiplied 

 by the time required to decelerate from Si to S 2 is equal 

 to the energy loss in deceleration from Si to S 2 , 



